In order to improve the fuel economy and power density of diesel engines while restricting exhaust emissions to within acceptable levels, fuel systems incorporating direct injection of the fuel into a combustion chamber (that is, the cylinder) of an engine through an injector, were developed. One of these direct injection fuel systems is the common rail fuel system. As high speed direct injection engines developed, energy to mix air with fuel in the cylinder was provided by the fuel spray momentum, as opposed to swirl mechanisms which were employed in the older combustion systems. Good mixing of fuel and air before combustion allows for even temperature distribution in the cylinder (that is, no hot spots), and reduced particulate matter, hydrocarbon and NOx emissions. To provide the mixing energy needed for low emissions from the engine, High Pressure Common Rail (HPCR) fuel injection systems were employed in diesel engines.
In an HPCR system, fuel is distributed to injectors, coupled to engine cylinders, from an accumulator, called the rail. Fuel from a fuel tank is delivered to the rail by a high pressure pump. The pressure of the fuel in the rail, as well as the start and end of injection in each cylinder (that is, injection rate and timing) are electronically controlled by a control system associated with the engine. In general, fuel pressure in a typical HPCR system does not depend on the engine speed and load conditions, thereby allowing for flexibility in controlling both injection rate and timing. Decoupling fuel pressure from engine speed allows for good fuel mixing even at low engine speeds and loads. In a typical HPCR system, a low pressure lift pump delivers fuel from a fuel tank through one or more fuel filters to the high pressure pump. An actuator operated metering valve controls the amount of fuel entering the high pressure pump. The high pressure pump delivers fuel at a constant pressure via a pressure regulator to the rail. Pressure sensors and temperature sensors installed in the fuel system monitor the temperature and pressure at critical points of the system. An Electronic Control Unit (ECU) may use these pressure and temperature readings, along with other inputs from the engine, to control the rail pressure, timing and duration of injection. The ECU may operate one or both the pressure regulator and the inlet metering valve of the high pressure pump to control the rail pressure. To initiate injection into a cylinder, the ECU may apply an electrical pulse having a variable duration to a control valve of the injector. The control valve may open a throttle in response to the electrical pulse, thereby fluidly coupling the rail to the cylinder. The duration of the electrical pulse may control the amount of time the control valve keeps the throttle open. Thus, the ECU may control the timing and duration of injection by controlling the timing and width of the electrical pulse used to energize the control valve. The pulse duration may determine the amount of fuel delivered to the cylinder. For a fixed pulse duration, the amount of fuel delivered into the cylinder increases with increased rail pressure. The ECU may determine the required amount of fuel to be delivered to the engine based on engine power output requirements. Engine power output requirements may be determined based on inputs such as engine speed, throttle position, etc. The ECU may determine the duration of the electrical pulse to be applied to the control valve based on the required amount of fuel and the rail pressure.
In a typical HPCR system, disruption of fuel flow on the upstream side of the high pressure pump may decrease the rail pressure. The decreased rail pressure may significantly increase the duration of time fuel may be sprayed into the cylinder to meet engine power output requirements. This increased spray time may prolong fuel injection well into the power stroke and decrease the effectiveness of combustion. In some cases, a prolonged fuel spray may also cause the injected fuel to miss the piston bowl and land on the cylinder liner instead. Liner fuel injection may cause reliability issues such as liner scuffing and wear, and detrimentally affect fuel economy and emissions.
U.S. Pat. No. 7,287,515 ('515 patent), a patent to Okamura et al., issued on Oct. 30, 2007, describes an engine fuel control system that modifies the quantity of fuel delivered to the engine based on an operating condition of the engine. The fuel control system of the '515 patent uses an engine controller to calculate a corrected fuel injection quantity based on an estimated rail pressure at a prescribed fuel injection time. The rail pressure is estimated based on a computed rail pressure correction factor. The fuel control system of the '515 patent delivers the corrected fuel injection quantity to a cylinder through an injector to produce a desired engine torque. Although the fuel control system of the '515 patent controls the quantity of fuel delivered to the engine based on the rail pressure, the system may still be susceptible to the previously described issues when the rail pressure is low.